The present invention relates to electrodes for a storage battery, and more particularly, to a positive nickel electrode and a negative electrode for an alkaline storage battery and manufacturing methods thereof, for use in a secondary battery such as an Ni-Cd, Ni-Fe, Ni-Zn, Ni-H or Ni-MH battery, which increase the capacity of the battery and reduce the self-discharge of the battery when open-circuited.
A positive nickel electrode for an alkaline storage battery is made by filling up, so as to permeate, a porous nickel current collector with an active material, i.e., nickel hydroxide, by a sintering or paste method.
In the fabrication of the positive electrode according to the sintering method, first, a porous nickel current collector is made by coating a nickel-plated steel plate with a slurry containing nickel powder as a main component, and drying and sintering the slurry-coated plate. Then, an active material containing nickel hydroxide is precipitated, chemically or electrochemically, at the pores of the nickel current collector, and treated in an alkaline solution. In this method, since the nickel current collector and the active material are strongly bonded and contact each other electrically over a large area, this type of positive nickel electrode exhibits the advantages of a high charging and discharging efficiency and a long life. Also, when additives to the active material are required, the amount of additives can be easily controlled by adding sodium nitrate containing a different element to a nickel nitrate solution and submerging the electrode in the solution.
The sintering method, however, is complicated to perform and costly. The maximum porosity of the current collector is no more than 80% and thus the density of the precipitated active material is relatively low.
On the other hand, a paste type positive nickel electrode is made by spraying or coating a porous nickel current collector of a strong alkali-proof foam metal with an active material in the form of paste and drying the current collector.
Such a paste type positive nickel electrode is advantageous over the sintering type positive nickel electrode in terms of process simplicity and thus fitness for mass production. However, since the porous nickel current collector is directly filled with the active material in the form of paste, the active material contacts the current collector over a smaller area than in the sintering type positive nickel electrode, thus lowering the performance of the battery.
FIG. 1 schematically illustrates a conventional alkaline battery and its positive electrode structure.
Referring to FIG. 1, a nickel porous body 11 formed on a positive electrode plate 10 is filled with particles 12 of an active material containing nickel hydroxide and additives. The active material particles 12 each are coated with a conductive layer 13 of, for example, Co(OH).sub.2. Reactions take place at the positive nickel electrode during charge and discharge, as follows: EQU Ni(OH).sub.2 +OH.sup.- .revreaction.NiOOH+H.sub.2 O+e.sup.-
The crystal structure of the nickel hydroxide, which depends on the manufacturing method thereof, experiences complicated changes during the reactions. Nickel hydroxide produced chemically in an aqueous solution is hexagonal .beta.-Ni(OH).sub.2, having a nickel ion interpositioned in an octahedral coordination between hydroxide ion layers.
.beta.-Ni(OH).sub.2 and .beta.-NiOOH, formed after charging, each have a c-axis length of about 4.6-4.8 .ANG., since other interstitial ions or H.sub.2 O are not introduced between the layers in the crystal structure. The charge and discharge reaction between .beta.-Ni(OH).sub.2 and .beta.-NiOOH results in little change in structure and volume, since hydrogen ions are merely adsorbed and dissociated between layers.
On the other hand, when .beta.-Ni(OH).sub.2 is overcharged, H.sub.2 O or interstitial ions are introduced between layers in .beta.-NiOOH, thereby producing .gamma.-NiOOH, which is changed into .alpha.-Ni(OH).sub.2 during discharging, in turn. Undesirable formation of this low-density .gamma.-NiOOH is accelerated when nickel hydroxide is filled in a high density to increase electron density, which obstructs diffusion of hydrogen ions into crystals.
These .gamma.-NiOOH and .beta.-Ni(OH).sub.2 have a c-axis length of about 7-8 .ANG., a 1.5 time-increase from that of .alpha.-Ni(OH).sub.2 or .beta.-NiOOH, since H.sub.2 O and interstitial ions between layers. Here, when charging, .alpha.-Ni(OH).sub.2 is changed into .beta.-Ni(OH).sub.2 of high density through chemical reaction, entailing a remarkable change in volume. This volume change causes swelling of the electrode and thus fall-off of the active material. As a result, a battery is charged in two stages.
It is known that the major causes of degradation of a positive nickel electrode is the swelling-induced fall-off of the active material, destruction of the current collector, and corrosion of nickel used for the current collector.
In an attempt to overcome the above problems, a method has been reported in which space for proton transfer is secured by transforming the lattice structure of nickel hydroxide. Thereby the conductivity of an active material is increased, leading to active transfer of electrons and suppression of .gamma.-NiOOH formation. For example, to induce transformation of the lattice, Zn or Mg is dissolved in a solid state in nickel hydroxide. To increase the conductivity, a conductive material such as a Co class compound is added. Co or preferably CoO is generally used as the conductive material. However, since the amount of the added compound reaches 10-20% of the total amount, the amount of nickel hydroxide used as the active material is relatively reduced, thus decreasing the capacity of the battery. Furthermore, it is difficult to completely prevent the electrode degradation and .gamma.-NiOOH formation caused during repeated charge and discharge processes.
In addition, in the charge and discharge reactions of the conventional battery, the change in oxidation number is only one through the reaction of .beta.-Ni(OH).sub.2 .revreaction..beta.-NiOOH. One electron per nickel atom is exchanged during charge and discharge. Thus, the theoretical capacity is merely 289 mAhr/g.
FIG. 2 schematically illustrates a conventional alkaline battery and its negative electrode structure. A negative electrode plate 20 for a cathode includes an active material structure 25 formed thereon and a hydrogen storage metal 26 filling the active material structure 25.
The hydrogen storage metal 26 itself is a metal, thus having no problem related to conductivity. However, its hydrogen storage capacity sensitivity varies with temperature, and thus the self-discharge of the battery easily occurs at high temperatures. Moreover, the conventional negative electrode using Cd(OH).sub.2 as the active material has the problem of large self-discharge.